Enzymatic methods using L-phenylalanine ammonia-lyase for the conversion of trans-cinnamic acid to L-phenylalanine generally comprise the steps of (a) aerobically propagating a phenylalanine ammonia-lyase (hereinafter PAL)-producing microorganism in an aqueous nutrient medium until substantial amounts of PAL are produced, (b) contacting the cells of the PAL-producing microorganism from step (a), either as the whole culture broth or separated cells therefrom, or the isolated enzyme, with ammonium ions and transcinnamate ions and allowing the reaction to proceed under controlled temperature and pH conditions until the conversion to L-phenylalanine is substantially complete and (c) separating and recovering the L-phenylalanine from the reaction mixture.
The foregoing method is described, for example, in British Pat. No. 1,489,468 (Oct. 19, 1977). A drawback to the use of this process for commercial production has been the relative instability of PAL, and its inhibition by the substrate, t-cinnamic acid. To drive the reaction toward the production of L-phenylalanine and to counteract the effects of substrate inhibition, the abovementioned British patent describes a process which employs large masses of PAL-containing cells and excess concentrations of ammonium ions.
Yamada, S. et al. (Appl. and Environ. Microbiol., 42, 7873-778 (1981)) have described the production of L-phenylalanine from t-cinnamic acid using PAL-containing Rhodotorula glutinis cells. They speculated that the lack of previous practical application of this process was attributable to the low activity and instability of microbial PAL. Yamada, et al. found that L-isoleucine had a stabilizing effect on PAL, and extended the useful period of activity of the enzyme. These authors further observed the inhibitive effect of the substrate, noting that at practical concentrations of t-cinnamic acid (150 mM), the rate of conversion of L-phenylalanine was reduced to one-half the maximum rate.
Despite the improvements described above, the instability and low activity of PAL has continued to be a disadvantage of this process. Accordingly, a need exists for procedures for stabilizing the enzyme and improving the
process for producing L-phenylalanine.